1. Field of the Invention
The present invention relates to oriented polyacetal products which have modified surfaces, particularly to such products having surfaces with improved affinity for other materials. The present invention also relates to a process for producing such products.
2. Discussion of the Related Art
By virtue of their high tensile strength and being superior in fatigue endurance, chemical resistance, and water repellency, polyacetals are used extensively as engineering plastics or as raw materials of synthetic fibers in various areas. In recent years, improvements of polyacetals of high elastic moduli and high strength have been developed, and as a result they have come to attention as materials suitable for composites with elastomers, plastics, or other materials.
However, because of the deficiency of polyacetals in their affinity for dissimilar materials, there have been difficulties in adhering polyacetals firmly to such materials, and when polyacetals are subjected to treatments such as plating, painting, coating, or printing, it is difficult to fix the applied material securely on their surfaces.
To overcome these difficulties, various surface treatments are have been practiced including; mechanical surface roughening with sandpaper or sandblasting; physical surface etching by corona discharge, plasma discharge, or the like; and chemical surface etching with potassium dichromate, phosphoric acid, fluorine, or other reagents. But, these treatments only have some adhesion-enhancing effect based on physical actions such as an anchor action, which hardly achieve any adhesion-enhancing effect based on chemical bonds. Therefore, the result of these treatments are not always satisfactory in the permanency of, peel resistance, or dynamic-fatigue resistance of the bond. The above treatments have also little adhesion-enhancing effect, because non crystalline portions that are relatively receptive to chemical etching are decreased by molecular orientation and hence these oriented polymers are virtually chemically inert.
On the other hand, British Patent No. 941,647 discloses a method comprising coating a polyacetal or other plastics to which it is to be adhered, with an unoriented polyacetal solution in a fluorine-containing organic solvent such as 2H-hexafluoro-2-propanol, and adhering the two materials together and evaporating the solvent from the applied solution to form an unoriented polyacetal coating. Also disclosed is a method comprising coating a metal or other materials to which a polyacetal is to be adhered, with such a solution as mentioned above, followed by evaporation. That is, these methods comprise interposing an unoriented polyacetal layer between a polyacetal and the intended adherent material. Besides 2H-hexafluoro-2-propanol, a number of organic solvents including phenols such as p-chlorophenol have been investigated up to now to dissolve polyacetals [J. Polym. Sci., Vol. 34, p. 185 (1959)]. According to the present inventors' study, however, the adhesive strength of roughened surfaces of unoriented polyacetals to rubbers, when the surfaces were coated with an unoriented polyacetal solution in the above-mentioned fluorine-containing organic solvent, or in a phenol, and dried, was decreased relative to the case where no such solution was applied. The same treatment of oriented polyacetals was found to decrease the adhesive strength similarly to the effect noted for the treatment of the unoriented polyacetals, and thus no useful effect from the treatment was observed. The use of adhesives of the epoxy resin type for the bonding also exhibited no adhesion-enhancing effect.
Moreover, none of the resorcinol-formaldehyde-rubber latex aged products, viz. RFL., and the initial stage condensate of resorcinol with formaldehyde viz. resol, which are well known as adhesives for rubber-reinforcing fiber cords and as adhesives for wood, exhibit any effect of enhancing the adhesive strength of polyacetals.
The reason why polyacetals are lacking, as described above, in affinity for dissimilar materials is that polyacetals, unlike nylons and cellulosics that are highly polar, have no functional group at the surface. Molecular orientation makes this tendency more noticeable.